1. Field of the Invention
This invention relates to an uninterruptible power supply with a battery.
2. Description of the Prior Art
FIG. 1 is a block diagram showing one example of a conventional uninterruptible power supply.
In FIG. 1, the reference numeral 50A designates an uninterruptible power supply which includes a commercial power source 1, a transformer 2 for a rectifier 3, the rectifier 3 having diodes, a smoothing inductor 4, a smoothing-capacitor 5, and an inverter 6. There are also provided an AC filtering-inductor 7 and an AC filtering-capacitor 8 for improving the waveform of the AC output of the inverter 6.
The uninterruptible power supply 50A also includes a battery 9 and a charging circuit 60A which includes a commercial power source 10, a transformer 11 for a charger 12, the charger 12 having thyristors and a smoothing inductor 13. There is further provided a thyristor switch 14 connected to the battery 9 for supplying DC power to the inverter 6 when the commercial power source 1 fails. However in the conventional uninterruptible power supply 50A shown in FIG. 1, it is necessary to provide the charge circuit 60A exclusively. Therefore while the uninterruptible power supply 50A shown in FIG. 1 is suitable for a large capacity type uninterruptible power supply, it is not suitable for use as small capacity type from an economical standpoint.
A conventional small capacity type uninterruptible power supply, commonly known as a "battery floating system" has been widely used, an example of which is shown in FIG. 2. In FIG. 2, like reference numerals designate identical or corresponding parts as those shown in FIG. 1. In FIG. 2, the reference numeral 50B designates an uninterruptible power supply, numeral 21 designates a transformer for a rectifier 22 which includes thyristors and simultaneously operates as a rectifier and a charger for the battery 9, numeral 23 designates a smoothing-inductor and numeral 24 designates a smoothing-capacitor. The rectifier 22 is controlled by changing the phase control angle a of the thyristors thereof by a control circuit (not shown) for either constant voltage control or constant current control of the output of the rectifier 22. Therefore the rectifier 22 provides a function of constant voltage charging or constant current charging of the battery 9 according to the charging status of the battery 9.
As described above, in the uninterruptible power supply 50B shown in FIG. 2, it is not necessary to provide an exclusive charge circuit such as the charge circuit 60A shown in FIG. 1. Thus, the uninterruptible power supply 50B has the advantage that the number of elements required for constructing the uninterruptible power supply is smaller than that of the uninterruptible power supply 50A shown in FIG. 1.
Also, it is noted that a battery is generally to be charged with a so-called equalizing charge voltage which is higher than the usual charge voltage (a floating charge voltage) by 10-15% thereof at regular intervals of a few months. Therefore it is necessary to design the transformer 21 and the rectifier 22 of the uninterruptible power supply 50B to be able to generate the equalizing charge voltage. When the rectifier 22 is required to generate the equalizing charge voltage, the phase control angle .alpha. of the thyristors of the rectifier 22 is to be controlled to a small value or zero.
On the contrary when the rectifier 22 is required to generate the floating charge voltage in an ordinary operation time of the uninterruptible power supply 50B, the phase control angle .alpha. of the thyristors of the rectifier 22 is to be controlled to a considerably large value. As a result of this, the input power factor in ordinary operation of the uninterruptible power supply 50B is reduced considerably, so the capacity of the transformer 21 and the input power capacity of the transformer 21 necessary for generating the floating charge voltage must be designed to be larger values than those of the transformer 2 shown in FIG. 1.
Furthermore the output ripple voltage of the rectifier 22 becomes larger, so both the smoothing-inductor 23 and the smoothing-capacitor 24 are to be provided with larger capacities.
As described above though the uninterruptible power supply 50B shown in FIG. 2 has the merit that the number of construction elements is small, it is not necessarily certain that this uninterruptible power supply 50B is superior from an economical standpoint. Therefore, this uninterruptible power supply has found limited use.